Organic light emitting devices (OLEDs) are comprised of several organic layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device, C. W. Tang et al., Appl. Phys. Lett 51, 913 (1987). Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995). Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor.
A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was reported in International Patent Application No. PCT/US95/15790. This TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg--Ag--ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mg--Ag--ITO electrode layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color. This colored emission could be transmitted through the adjacently stacked transparent, independently addressable, organic layer, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers.
The PCT/US95/15790 application disclosed an integrated OLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. The PCT/US95/15790 application, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
Such devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of injected charge. Specifically, OLEDs are comprised of at least two thin organic layers separating the anode and cathode of the device. The material of one of these layers is specifically chosen based on the material's ability to transport holes (a "hole transporting layer") and the material for the other layer is specifically selected according to its ability to transport electrons (an "electron transporting layer"). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the hole transporting layer, while the cathode injects electrons into the electron transporting layer. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. Recombination of this short-lived state may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism. Under this view of the mechanism of operation of typical thin-layer organic devices, the electroluminescent layer comprises a luminescence zone receiving mobile charge carriers (electrons and holes) from each electrode.
The materials that function as the electron transporting layer of the OLED are frequently the same materials that are incorporated into the OLED to produce the electroluminescent emission. Such devices are referred to as having a single heterostructure. Alternatively, the electroluminescent material may be present in a separate emissive layer between the hole transporting layer and the electron transporting layer in what is referred to as a double heterostructure.
In order to be useful in practical, low cost, active matrix displays, OLED device structures that are integratable with pixel electronics are required. A conventional OLED is grown on a transparent anode such as ITO, and the emitted light is viewed through the substrate, complicating integration with electronic components such as silicon-based display drivers. It is therefore desirable to develop an OLED with emission through a top, transparent contact.
A surface-emitting polymer-based OLED grown on silicon with a transparent ITO and a semitransparent Au or Al top anode has been demonstrated, D. R. Baigent, et al., Appl. Phys. Lett. 65, 2636 (1994); H. H. Kim, et al., J. Lightwave Technol. 12, 2107 (1994). A similar integration of molecular OLEDs with silicon was achieved using a tunneling SiO.sub.2 interface, Kim et al. The tunneling interface, however, increases the device operating voltage, and can be avoided in structures such as for the recently reported transparent TOLEDs, V. Bulovic, et al., Nature 380, 29 (1996) and G. Gu, et al., Appl. Phys. Lett. 68, 2606 (1996), which can, in principle, be grown on a silicon substrate. The TOLED anode, however, forms the electrode contact which is in direct contact with the substrate, "the bottom contact", whereas for display drivers employing n-channel field effect transistors (NFETs), such as amorphous silicon NFETs, it is desirable for the bottom contact of the OLED to be the cathode. This requires fabricating inverted OLEDs (IOLEDs), that is, devices in which the order of placing the sequence of layers onto the substrate is reversed. In particular, for a single heterostructure IOLED, the electron-injecting cathode layer is deposited onto the substrate, the electron transporting layer is deposited onto the cathode, the hole transporting layer is deposited onto the electron transporting layer and the hole-injecting ITO anode layer is deposited onto the hole transporting layer.
Fabrication of such IOLEDs, or TOLEDs, thus requires that the ITO anode layer be sputter-deposited directly or indirectly onto relatively fragile organic thin films. Since the ITO layer is typically deposited using conventional sputtering or electron beam methods so as to produce layers having a thickness from about 500 .ANG. (angstroms) up to as much as 4000 .ANG., it is desirable that these layers be deposited at the highest deposition rates possible so as to reduce the time necessary to prepare such layers. Unfortunately, it has been found that while the ITO layer may be deposited at a rate of up to 50-100 .ANG. per minute, or more, whenever the ITO layer is deposited on a bare substrate, the deposition rate may be only about 2-5 .ANG./minute, if the ITO is deposited directly onto an organic layer or onto a Mg:Ag surface, which is itself typically deposited over several organic layers. Since the organic layers are highly vulnerable to damage when subjected to the beam of high energy particles that are used at the higher ITO deposition rates, higher deposition rates can cause substantial damage to the underlying organic layers, thus causing unacceptably large deterioration in the overall performance of the OLED.
Whenever the ITO layer of an OLED is deposited onto such fragile organic surfaces, it would be desirable if the ITO layer could be deposited at substantially higher deposition rates than have been possible until now.